The mobile messaging service WhatsApp, acquired last week by Facebook for $19 billion, has said it will offer free voice calling for its 465 million users as early as April.

The WhatsApp voice calling service would “focus on simplicity,” said the company’s chief executive Jan Koum, at the Mobile World Congress conference in Barcelona, Spain. The service would first be released for the iPhone and the Android, followed by Microsoft’s Windows Phone, and some Blackberry and Nokia phones, reports the Wall Street Journal.

“We want to continue to have that minimalistic approach to the product,” Koum said. “We at WhatsApp want to get out of the way and let people communicate.”

WhatsApp will remain independent after its Facebook acquisition, Koum said, citing Instagram as an example after its acquisition by Facebook. Koum said he is aiming to grow WhatsApp service to one or two billion users.